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Flowering Plants; They Were Too Numerous and Too Varied, and There Were Too Few Fos- Sils to Sort out Which Were More Primitive

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Flowering Plants; They Were Too Numerous and Too Varied, and There Were Too Few Fos- Sils to Sort out Which Were More Primitive NEWSFOCUS embryo that serves as its food supply. Darwin was perplexed by the diversity of On the Origin of flowering plants; they were too numerous and too varied, and there were too few fos- sils to sort out which were more primitive. Flowering Plants Throughout much of the 20th century, mag- nolia relatives with relatively large flowers were leading candidates for the most primi- how flowers got started—and from which tive living flowers, although a few ancestor. Today, researchers have analytical researchers looked to small herbs instead. tools, fossils, genomic data, and insights that In the late 1990s, molecular systematics Darwin could never have imagined, all of came to the rescue, with several reports pre- which make these mysteries less abom- senting a fairly consistent picture of the inable. Over the past 40 years, techniques lower branches of the angiosperm tree. An for assessing the relationships between obscure shrub found only in New Caledonia organisms have greatly improved, and gene emerged as a crucial window to the past. sequences, as well as morphology, now help Amborella trichopoda, with its 6-millimeter researchers sort out which angiosperms greenish-yellow flowers, lives deep in the arose early and which arose late. New fossil cloud forests there. In multiple gene-based finds and new ways to study them—with assessments, including an analysis in 2007 synchrotron radiation, for example—pro- of 81 genes from chloroplast genomes vide a clearer view of the detailed anatomy belonging to 64 species, Amborella sits of ancient plants. And researchers from var- at the base of the angiosperm family tree, ious fields are figuring out genomic changes the sister group of all the rest of the that might explain the amazing success of angiosperms. this fast-evolving group. Given that placement, Amborella’s tiny on September 9, 2009 These approaches have given researchers flowers may hint at what early blossoms a much better sense of what early flowers were like. It’s one of “the most similar living IN 1879, CHARLES DARWIN PENNED A LETTER were like and the relationships among them. flower[s]” to the world’s first flower, says to British botanist Joseph Dalton Hooker, But one of Darwin’s mysteries remains: the James Doyle of the University of Califor- lamenting an “abominable mystery” that nature and identity of the angiosperm ances- nia, Davis. The petals and sepals of its sin- threw a wrench into his theory of evolution: tor itself. When flowering plants show up in gle-sex flowers are indistinguishable and How did flowering plants diversify and the fossil record, they appear with a bang, vary in number; so too do the numbers of spread so rapidly across the globe? From with no obvious series of intermediates, as seed-producing carpels on female flowers www.sciencemag.org rice paddies to orange groves, alpine mead- Darwin noted. Researchers still don’t know and pollen-generating stamens on male ows to formal gardens, prairies to oak- which seed- and pollen-bearing flowers. The organs are spirally hickory forests, the 300,000 species of organs eventually evolved into THE YEAR OF arranged, and carpels, rather angiosperms alive today shape most terres- the comparable flower parts. than being closed by fused tis- trial landscapes and much of human life “We’re a bit mystified,” says DARWIN sue as in roses and almost all and culture. Their blooms color and scent botanist Michael Donoghue of familiar flowers, are sealed by a our world; their fruits, roots, and seeds feed Yale University. “It doesn’t secretion. us; and their biomass provides clothing, appear that we can locate a close Most genetic analyses showed Downloaded from building materials, and fuel. And yet this relative of the flowering plants.” that water lilies were the next takeover, which took place about 100 mil- branch up the angiosperm tree, lion years ago, apparently happened in a Seeking the first flower followed by a group represented blink of geological time, just a few tens of One of two major living groups by star anise, which also has a millions of years. of seed plants, angiosperms have primitive look about it, says The father of evolution couldn’t quite “covered” seeds that develop This essay is the fourth Doyle, “though each of these fathom it. Darwin had an “abhorrence that encased in a protective tissue in a monthly series. has deviations from the ances- For more on evolutionary evolution could be both rapid and poten- called a carpel (picture a bean topics online, see the tral type.” tially even saltational,” writes William pod). That’s in contrast to the Origins blog at Friedman in the January American Journal nonflowering gymnosperms, blogs.sciencemag.org/ Fossil records origins. For more on of Botany, which is devoted to this such as conifers, which bear flower origins, listen Although some fossil pollen “abominable mystery.” Throughout his life, naked seeds on scales. An to a podcast by author dates back 135 million years, no Elizabeth Pennisi at Darwin pestered botanists for their angiosperm’s carpel sits at the www.sciencemag.org/ credible earlier fossil evidence thoughts on the matter, but they couldn’t center of the flower, typically multimedia/podcast. exists. In Darwin’s day, and for give him much help. surrounded by pollen-laden sta- many decades afterward, pale- Now, 130 years later, evolutionary biol- mens. In most flowers, the carpel and stamens obotanists primarily found leaves or pollen ogists are still pestering botanists for clues are surrounded by petals and an outer row of but almost no fossil flowers. They had the about what has made this plant group so leaflike sepals. Seeds have a double coating wrong search image, says Else Marie Friis of successful, as well as when, where, and as well as endosperm, tissue surrounding the the Swedish Museum of Natural History in LONDON (SUPERSTOCK) LIBRARY, GEORGE RICHMOND/BRIDGEMAN ART GARDENS, KEW; BOTANIC CREDITS (TOP TO BOTTOM): REPRODUCED WITH THE KIND PERMISSION OF DIRECTOR AND BOARD TRUSTEES, ROYAL 28 3 APRIL 2009 VOL 324 SCIENCE www.sciencemag.org Published by AAAS ORIGINS Stockholm. “When we started, the search profile was bigger, a magnolia [flower],” she recalls. But 30 years ago, she and others discovered tiny ancient flowers by sieving through sand and clay sedi- ments. With this technique, they have now collected hun- dreds of millimeter-size flowers, some preserved in three dimensions, from Por- tugal and other locations with Cretaceous deposits 70 mil- lion to 120 million years old. This fossil diversity shows that angiosperms were thriving, with several groups well-established, by 100 mil- Out of the past. lion years ago. In some, the Tiny Amborella sits flower parts are whorled like at the bottom of the those of modern flowers; in angiosperm family tree. others they are spiraled, con- sidered by some researchers as the more primitive arrangement. Some from one of the nonflowering seed plants flower fossils have prescribed numbers of “We are realizing that this or gymnosperms, whose heyday was 200 on September 9, 2009 petals, another modern feature, whereas in million years ago. Modern gymnosperms others the petal count varies. huge diversity is probably include conifers, ginkgoes, and the cycads, In 1998, Chinese geologist Ge Sun of with their stout trunks and large fronds. Jilin University in Changchun, China, came the result of one innova- Before angiosperms came along, these across what seemed to be a much older plants were much more diverse and flower. The fossil, called Archaefructus, was tion piled on top of included cycadlike species, such as the an aquatic plant that looked to be 144 mil- another innovation.” extinct Bennettitales, and many woody lion years old. By 2002, Sun and David plants called Gnetales, of which www.sciencemag.org Dilcher of the Florida Museum of Natural —Peter Crane, a few representatives, including the History (FLMNH) in Gainesville had University of Chicago joint firs, survive today (see family tree, described an entire plant, from roots to flow- p. 31). Also common in the Jurassic were ers, entombed on a slab of rock unearthed in These fossils often spark debate because seed ferns, a group now long gone; their Liaoning in northeastern China. specimens tend to be imperfectly preserved most famous member is Caytonia, which In one sense, Archaefructus wasn’t much and leave room for interpretation. To help seems to have precarpel-like structures. to look at. “It’s a flowering plant before remedy that, Friis and her colleagues have These groups’ perceived relevance to there were flowers,” Dilcher notes. It lacked begun to examine flowers using synchro- flower evolution and their relationships to Downloaded from petals and sepals, but it did have an tron radiation to generate a 3D image of angiosperms have ping-ponged between enclosed carpel. When Kevin Nixon and their inner structures, allowing the fossil to camps, depending on how the evolutionary colleagues at Cornell University compared remain intact while Friis peers inside it trees were constructed. its traits with those same traits in 173 living from many angles (Science, 7 December In the mid-1980s, Peter Crane, now at plants, Archaefructus came out as a sister to 2007, p. 1546). “We can get fantastic reso- the University of Chicago in Illinois, pro- living angiosperms and closer to the com- lution,” says Friis. “It’s really exciting.” But posed a solution, the anthophyte hypothesis. mon ancestor than even Amborella. so far, the flowers Friis finds are Using several lines of evidence and noting Archaefructus’s distinction was short- too diverse to trace back to a that both Bennettitales and lived, however. Within months, better dat- particular ancestor. “From Gnetales organize their male ing of the sediments in which it was found these fossils, we cannot and female organs together yielded younger dates, putting this first say what is the basic in what could be con- flower squarely with other early fossil form,” she says.
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